The objective of this research project is to provide a better understanding of the genetically determined factors which influence the immunogenicity of alloantigens present on transplanted tissue. As a model system we will use the Thymus cell antigen-1 (Thy-1) cell surface antigens of the mouse. Because the genes of the tissue donor which confer augmented immunogenicity upon Thy-1 antigens are expressed in a dominant or condominant manner, F1 hybrids between highly immunogenic and poorly immunogenic inbred strains show elevated Thy-1 immunogenicity. By backcrossing such hybrid to the poorly immunogenic strain we will estimate the number of augmenting or "helper" loci. The segregation patterns of these loci will be compared with those of previously characterized genes in an attempt to define their chromosomal locations. Serial backcrossing of the hybrids to the poorly immunogenic parental strain should allow separation of the individual "helper" genes so that their relative strengths can be compared and their abilities to augment immune responses against both the known Thy-1 antigenic variants can be assessed. We will also examine the question of whether the H-Y minor histocompatibility antigen can act as a "helper" determinant, thereby enhancing the immunogenicity of associated Thy-1 molecules.